Method for manufacturing optical fibre preforms

ABSTRACT

Multi-flame burner wherein each flame is separated with respect to the neighboring flame by at least one separating tube made of a heat resistant material, for example, quartz glass or ceramic material. The burner also has a plurality of co-axial pipes, preferably made of a metallic material. The cross section of the upper end of the separating tube can be modified in order to increase the deposition rate of the burner. Methods for manufacturing optical fibre preforms by vapour deposition using the multi-flame deposition burners.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP01/14016, filed Nov. 30, 2001, the content of which isincorporated herein by reference, and claims the priority of EuropeanPatent Application No. 00127851.4, filed Dec. 19, 2000, and claims thebenefit of U.S. Provisional Application No. 60/256,942, filed Dec. 21,2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a burner for manufacturing an opticalfiber preform used to make optical glass fibers and to a method forproducing said optical, preforms.

In particular, the present invention relates to a multi-flame depositionburner suitable for manufacturing optical preforms in an efficient andstable manner at high fabrication rate and to a method for producing anoptical preform by using said burner.

2. Background Art

Glass fibers for optical communication are made from high purity,silica-based glass fibers drawn from glass preforms, which preforms areproduced according to various glass deposition techniques.

Some of these deposition techniques, including vapor axial deposition(VAD) and outside vapor deposition (OVD), are based on flame combustionwherein reactants (i.e. silica precursors, such as SiCl₄, optionallytogether with dopants materials, such as GeCl₄, for suitably modifyingthe refractive index of the glass) are fed together with combustinggases through a deposition burner which directs a high temperature flowof forming fine glass particles onto a rotating growing target preform.

According to the VAD deposition technique, the growth of the preformtakes place in an axial direction. Thus, the deposition burner(s) istypically maintained in a substantially fixed position, while therotating preform is slowly moved upwardly (or downwardly) with respectto the burner, in order to cause the axial growth of the preform.Alternatively, the rotating preform can be maintained in a substantiallyfixed position, while the deposition burner is slowly moved downwardly(or upwardly) with respect to the preform.

Differently from the VAD technique, in the OVD technique the growth ofthe preform takes place in a radial direction. In this case, a rotatingtarget (e.g. a quartz glass rod) is generally positioned in a fixedhorizontal or vertical position and the deposition burner is repeatedlypassed along the surface of the growing preform for causing the radialgrowth of the same.

Independently from the applied deposition technique, a porous glasspreform is thus fabricated, which is then consolidated to form a solidglass preform apt for being subsequently drawn into an optical fiber.

Typically, an optical preform comprises a central portion (core) and anouter portion (cladding), the core and the cladding differing in theirrespective chemical composition and having thus different refractiveindexes. As in the optical fibers, the cladding portion forms themajority of the preform. The preform is typically manufactured byproducing and consolidating a first preform comprising the core and afirst portion of the cladding. An overcladding layer is then depositedonto said first preform, thus obtaining a porous preform, which is thenconsolidated into the final preform.

In general, conventional burners for manufacturing optical preforms aremade up of a plurality of co-axial pipes through which the glassprecursor materials (i.e. silica precursors, such as SiCl₄, optionallytogether with dopants materials, such as GeCl₄), the combusting gases(e.g. oxygen and hydrogen or methane) and, optionally, some inert gas(e.g. argon or helium) are fed. Typically, the glass precursor materialis fed through the central pipe of the burner, while other gases are fedthrough the annular openings formed by the concentrically disposedpipes.

Examples of such conventional burners are disclosed, for instance, inU.S. Pat. Nos. 4,345,928, 4,465,708, 4,474,593, 4,661,140, and4,810,189.

“Multi-flame” burners, generating a plurality of independent flamesdisposed concentrically one to each other, are also disclosed. Forinstance, U.S. Pat. No. 4,801,322 discloses a multi-flame burner whereinthe inner flame, including a glass precursor material, is positionedrearwardly of the outer flame. As mentioned in said patent, the outerflame allows to increase the flame length with consequent size increaseof the synthesised glass particles.

U.S. Pat. No. 4,826,520 discloses a modified multi-flame burner forproducing doped optical preforms wherein a central pipe, through which adoping reactant (GeCl₄) is fed, is spaced forwardly with respect to theother pipes forming the inner flame, in order to reduce the staying timeof the doping material inside the flame.

Although few prior art documents disclose burner having pipes made froma heat resistant metallic cylindrical material (e.g. U.S. Pat. No.4,661,140), the pipes of conventional prior art burners are generallyand more desirably made from quartz glass or ceramic materials, asdisclosed for instance in U.S. Pat. No. 4,345,928 (col. 8, lines 52-55),U.S. Pat. No. 4,474,593 (col. 2, lines 16-19), U.S. Pat. No. 4,465,708(col. 1, lines 58-61), U.S. Pat. No. 4,801,322 (col. 26, lines 32-40)and U.S. Pat. No. 4,810,189 (col. 4, lines 66-68).

As a matter of fact, quartz or ceramic materials are more heat resistantthan metallic materials to high temperatures and may thus more easilywithstand the typical temperature developed by the flame in the burner.In any case, the possible use of heat resistant metallic pipes inconventional deposition burner is necessarily limited to thesingle-flame type burners (such as the one disclosed in U.S. Pat. No.4,661,140). In these burners, all the co-axial pipes through whichreactants/inert gases flow have in fact substantially the same length;the overheating of said pipes is thus avoided by maintaining the flamesufficiently spaced apart from the tips of the pipes.

However, as observed by the Applicant, in the multi-flame burners of theprior art, such as the one disclosed in U.S. Pat. No. 4,801,322,problems may arise in using metallic materials for manufacturing thepipes of the burner. In fact, as disclosed in the above cited patent,the pipes generating outer flame are longer than the pipes generatingthe inner flame, in order to obtain the rearward spacing of the innerflame with respect to the outer flame. Thus, the inner surface of thepipes forming the outer flame is subjected to the heat generated by theinner flame. While the typical temperature of a flame is of about2500-3000° C., the surface of the pipes exposed to the flame may reach atemperature of several hundreds degrees, typically of about 600-800° C.As it is apparent that a pipe made from a metallic material can notwithstand the heat generated by such a flame, it is therefore necessary,as mentioned in the above cited patent, to use a burner with quartzglass pipes. This problem is much more evident for burners specificallydesigned for the outer cladding deposition, which produce larger flamesand accordingly higher amount of heat.

The Applicant has however observed that the use of quartz glass orceramic materials for producing a burner results in a number ofdrawbacks. In particular, the concentricity of glass pipes is rathercumbersome to obtain and particular attention shall be paid to therelative alignment of the co-axial pipes. In addition, a burnercontaining a plurality of glass pipes shall be handled with care foravoiding possible damages of the pipes.

The Applicant has now found that in a multi-flame burner, comprising atleast an inner section comprising a first plurality of ducts forgenerating an inner flame and at least an outer section comprising asecond plurality of ducts for generating an outer flame surrounding saidinner flame, said inner flame can be advantageously confined andseparated from the outer flame by disposing a separating tube made of aheat resistant material, in particular of quartz glass or ceramicmaterial (e.g. alumina), between said inner and said outer section.

According to such a burner design, the pipes forming the ducts of themulti-flame burner may thus advantageously be made from a metallicmaterial, e.g. stainless steel.

In addition, as observed by the Applicant, while the burners fordepositing the core and the inner cladding of the preform are generallyof reduced dimensions, the burner used for depositing the overcladding,in particular for large dimensions preforms, shall be relatively larger,in order to allow the generation of higher flow rates which arenecessary for increasing the amount of deposited material, maintainingat the same time the velocity of the gases relatively low.

The Applicant has thus observed that, particularly for overcladdingdeposition and especially when manufacturing large diameter opticalpreforms, the deposition rate of the process can be increased bysuitably modifying the geometry of deposition burner in order toredistribute the flow of fine glass particles impacting onto the targetpreform. In particular, it has been observed that the shape of said flowcan be advantageously modified in its terminal portion before impactingonto the target preform, by increasing the dimension of said flow in adirection substantially perpendicular to the longitudinal axis of saidtarget preform.

The modification of the geometry of the flow of glass particles isparticularly easy and effective when applied onto a multi-flame burnerwith a. single flame-separating tube according to the present invention.

SUMMARY OF THE INVENTION

One aspect of the present invention thus relates to a multi-flame burnerfor manufacturing an optical fiber preform comprising:

-   -   at least an inner section for generating an inner flame, said        inner section comprising a first plurality of ducts through        which at least a glass precursor material, a combustible gas and        a combustion sustaining gas are flown; and    -   at least an outer section for generating an outer flame        surrounding said inner flame, said outer section comprising a        second plurality of ducts through which at least a combustible        gas and a combustion sustaining gas are flown; wherein    -   an elongated hollow separating element made of a heat resistant        material is disposed to surround said inner section and prolongs        farther from the end of the ducts forming the outer section, in        order to confine said inner flame and separate said inner flame        from said outer flame.

Preferably, said first plurality of ducts disposed in the inner sectionof the burner is made from a metallic material. Advantageously, also thesecond plurality of ducts disposed in the outer section of the burner ismade from a metallic material.

According to a preferred embodiment, a second elongated hollow elementmade of heat resistant material is disposed to surround the outersection of the burner for containing the outer flame.

Preferably, said inner and said outer section of the burner are ofsubstantial circular form. Advantageously, said inner and said outersection of the burner are formed from a plurality of co-axial pipes madeof metallic material.

According to a preferred embodiment, the separating element is anelongated pipe of a heat resistant material, preferably of quartz glassor ceramic material, such as alumina.

Preferably, said separating elongated hollow element extends for alength such as to surround the majority of the length of the reactionzone of the glass precursor material.

Preferably, said separating elongated hollow element extends for alength of about at least 80 mm from the tips of the ducts forming theinner section of the burner. Preferably, said separating elongatedhollow element extends for a length of less than about 150 mm from thetips of the ducts forming the inner section of the burner.

According to an embodiment, the length of the ducts forming the innerand the outer section of the burner are substantially the same.

Advantageously, the inner section of the burner is spaced rearwardly ofthe outer section. Preferably, the pipes forming the outer section arefrom about 30 mm to about 80 mm longer than the pipes forming the innersection of the burner, more preferably from about 40 mm to about 65 mm.

According to a preferred embodiment, the elongated hollow elementseparating the two portion of the burner has an upper terminal portionwhich is formed into an elliptical cross-section.

A further aspect of the present invention relates to a method formanufacturing an optical preform by directing a flow of fine glassparticles from a deposition burner onto a rotating elongated targetpreform, said method comprising the steps of:

-   -   feeding said burner with a flow of a glass precursor material        and directing said flow of glass precursor material towards said        target preform;    -   feeding said burner with a first flow of combustible gas and a        first flow of combustion sustaining gas in order to generate an        inner flame surrounding said flow of glass precursor material;    -   feeding said burner with a second flow of combustible gas and a        second flow of combustion sustaining gas in order to generate an        outer flame surrounding said inner flame    -   causing said glass precursor material to react in the presence        of said flames, thus forming a flow of fine glass particles        directed towards said target preform;        wherein said first flame is confined and separated from said        second flame by an elongated hollow body disposed to surround        said inner flame.

A further aspect of the present invention relates to a method formanufacturing an optical preform by directing a flow of fine glassparticles from a multi-flame deposition burner comprising a plurality ofducts onto a rotating elongated target preform by using a multi-flamedeposition burner as above described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic transversal cross-sectional view of anembodiment of a burner according to the present invention;

FIG. 2 shows a schematic longitudinal cross-sectional view of anembodiment of a burner according to FIG. 1;

FIG. 3 shows a schematic transversal cross-sectional view of analternative embodiment of a burner according to FIG. 1;

FIG. 4 shows a schematic transversal cross-sectional view of analternative embodiment of a burner according to the present invention;

FIG. 5 shows a longitudinal cross-sectional view of a preferredembodiment of a burner according to the present invention;

FIG. 6 is a section according to line VI-VI of FIG. 5; and

FIG. 7 schematically shows an overcladding deposition process accordingto the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic transversal cross-sectional view of an exampleof a burner according to an embodiment of the invention. In thespecific, this embodiment illustrates a double-flame burner,particularly suitable for overcladding deposition.

The burner of FIG. 1 comprises seven openings 100 a-107 a through whichthe gases for forming the preform are passed.

Openings 101 a-103 a define the inner section of the burner, whileopenings 4 a-7 a define the outer section. The central opening 101 a isdelimited by the walls of a metal pipe 101, while the other annularopenings are delimited by the respective outer and inner walls of metalpipes 101 to 108. The length of the metal pipes can be substantially thesame, as shown in detail in FIG. 2, or the pipes defining the openingsof the outer section can preferably be longer than the pipes definingthe openings of the inner section, as shown in FIG. 3. However, in orderto avoid excess overheating of the pipes forming the outer section, thelength of said pipes shall preferably not exceed the length of the pipesof the inner section for more than about 80 mm, more preferably for morethan about 60 mm.

The metal pipes are preferably made from an easily machinable andheat/corrosion resistant stainless steel. An example of a suitable metalmaterial is AISI (American Institute Steel and Iron) 316L, which is astainless steel comprising about 0.03% C about 16-18% of Cr, about11.5%-14.5% of Ni, about 2% of Mn and about 2.5%-3% of Mo.

Typically, the inner pipe 101 has an inner diameter of from about 6 mmto about 8 mm and a thickness of from about 0.5 mm to about 2 mm.

The other metal pipes, having preferably a thickness comprised fromabout 0.5 mm to about 2.5 mm, are then arranged coaxially one to eachother to form openings 102 a-107 a having widths of from about 1 mm toabout 3.5 mm, depending on the relative diameter of the pipe and flowrate of gas through the aperture.

In particular, the width of each opening is selected according to theamount and kind of gas which is flown through said opening and to therelative radial position of said opening. For instance, in a burnerparticularly designed for the outer cladding deposition, openingsthrough which inert gas is flown are dimensioned so to obtain an exitvelocity of the gas of from about 0.1 and about 2 m/s. Said annularopenings may thus have a width of from about 1 mm to about 1.5 mm. Onthe other side, openings through which combustion gases are flown aredimensioned so to obtain an exit velocity of the gas of from about 2 andabout 10 m/s. Said annular openings may thus have a width of from about2 mm to about 3.5 mm.

A separating tube 109 made from a heat resistant material, is disposedinto the annular housing between pipes 103 and 104, said tube extendingfor a certain length farther from the tips of the pipes of the innerportion of the burner, as shown in FIGS. 2 and 3.

Tube 109 allows both to confine the inner flame for a certain length andto physically separate it from the outer flame. In addition, when thepipes forming the outer section of the burner are longer than the pipesforming the inner section, said tube 109 avoids a direct contact of theinner flame with the surface of the innermost metal pipe of the outersection.

A second tube 110, also made from heat resistant material, can bedisposed externally to the metal pipe 108, extending for a certainlength farther from the tips of the pipes of the outer portion of theburner, as shown in FIGS. 2 and 3, for confining the outer flame.

For the purposes of the present invention, the term “heat resistantmaterial” is intended to refer to a material capable of resisting totemperatures typically present in a deposition burner during a preformmanufacturing process, without undergoing to physical or chemicaldamages.

The heat resistant material of tubes 109 and 110 is for instance quartzglass or ceramic material, such as alumina. Preferably quartz, inparticular high purity quartz, is employed.

Preferably, the separating tube 109 extends for a length such tosurround the majority of the length of the reaction zone where the glassprecursor material reacts to form the glass particles.

Methods for approximately calculating the extension of the reaction zoneare well known and widely discussed in several reference books, such asK. K. Kuo, “Principles of Combustion”, Wiley and Sons Ed., New York.1986, p. 370.

An example of such calculation is given hereinafter, with specificreference to a burner having the following configuration:

-   -   a central duct through which SiCl₄ as glass precursor material        is flown;    -   a first annular duct (surrounding said central duct) through        which hydrogen is flown; and    -   a second annular duct, surrounding said first annular duct,        through which oxygen (in stoichometric excess with respect to        hydrogen) is flown.

As an approximation, it is assumed that all the hydrogen instantaneouslyreacts with oxygen at the outlet of the ducts. A flow of water andoxygen, surrounding the central flow of silicon tetrachloride, is thusformed. The glass precursor material reacts with the formed water toform silica, according to the following reaction:SiCl₄+2H₂O→SIO₂+4HCl

The length of the reaction zone can thus be calculated by applying thefollowing relationship:

$L = {\frac{3}{8\pi}\frac{1}{f_{st}}\frac{\pi\; r_{0}^{2}u_{o}}{\upsilon}}$

where

-   -   r₀ is the diameter of the outlet of the central duct;    -   u₀ is the velocity of the flow of silicon tetrachloride;    -   υ₀ is the viscosity of the reacting mixture; and

$f_{st} = \frac{\left( {F/O} \right)_{st}Y_{O,A}}{1 + {\left( {F/O} \right)_{st}Y_{o,A}}}$

-   -    where    -   F/O is the mass ratio of silicon tetrachloride and water under        stoichometric reaction conditions; and    -   Y_(O,A) is the mass fraction of water in the oxygen+water flow.

The separating tube may thus preferably have a length substantiallyequivalent to the calculated length of the reaction zone. Said tube maybe up to about 50-60% longer than said calculated length. Longer lengthsof the tube, e.g. 60% or more with respect to the calculated length ofthe reaction zone (in particular more than about 70%), while notsubstantially increasing the deposition rate of the burner, mayconversely negatively affect the deposition process. For instance, ifthe separating tube is excessively long and the burner is kept too closeto the target preform, the deposited soot can be subjected toundesirable local overheating, with consequent formation of cracks inthe soot. On the other side, if the burner is too spaced from thepreform in order to avoid the above overheating drawbacks, an irregulargrowth of the silica glass particles may occur, with consequentreduction of the deposition rate.

Thus, particularly for overcladding burners, depending from thedimensions of the ducts and the flow rate and velocity of the gasesflowing therethrough, the Applicant has determined that the separatingtube 109 should preferably extend for at least about 80 mm from the tipsof the pipes of the inner section of the burner. The length of the tubeshould however preferably not exceed about 150 mm. Preferably, saidlength is from about 90 to about 130 mm. When the pipes forming theinner section of the burner are spaced rearwardly with respect to thepipes of the outer section, the separating tube preferably extends forat least about 40 mm from the tips of the pipes of the outer section,more preferably for at least 50 mm, up to about e.g. 100 mm, preferably85 mm.

The outer tube 110 preferably extends for about 150 mm to about 220 mmfrom the tips of the pipes of the outer section.

Advantageously, the metallic coaxial pipes are first assembled togetherto form the burner, leaving a suitable annular clearance between twoneighbouring pipes, said clearance being apt to receive the separatingquartz tube 109. The separating tube 109 can thus be inserted into and(if necessary) removed from said annular clearance with a rather simpleoperation. Similarly, the outer glass tube 110 can be fitted on (andremoved from) the outer surface of the outer metal pipe (suitablyadapted for receiving said glass tube), after the metal pipes of theburner have been assembled together.

With the above construction, the size and concentricity of the metalco-axial pipes forming the burner can be controlled much more easierthan in conventional multi-flame burners where the co-axial pipes aremade from quartz glass. In addition, the manufacturing, handling andmaintenance of such burner can be performed in a rather simple manner,without the risk of breaking the pipes. Only a single glass tube is usedfor confining the inner flame and separating it from the outer one,which tube can be easily fitted into the burner after the whole burnerhas been assembled and (if necessary) removed from it, for instance incase of accidental breakage of the same.

Although the heat resistant materials from which tubes 109 and 110 aremade can withstand rather easily the typical temperatures developed bythe flames of the burner, it may be desirable to reduce the heattransmitted from the flames towards the surfaces of the heat resistanttubes.

Thus, in order to lower the heat transmitted from the flames towards thesurfaces of heat resistant tubes 109 and 110, a gas is preferablyallowed to flow along the internal and/or external walls of tube 109 andpreferably also along the internal wall of tube 110, to create aboundary layer on the respective surfaces of the tubes. The presence ofsuch boundary layer may in fact contribute to dissipate the heatgenerated by the flames, thus avoiding possible overheating of thetubes. Preferably, said boundary layer is formed by a laminar flow ofgas. To this end, any gas capable of forming said boundary layer at therelevant flow rates applied during the deposition operations can beemployed. Preferably, oxygen or inert gases, such as argon, helium ornitrogen, are employed.

Typically, the central duct 101 of a multi-flame burner according to thepresent invention is fed with a flow of glass precursor material,optionally admixed with a high thermal diffusivity gas. In the presentdescription, the term glass precursor material is intended to refer toany suitable material capable of reacting in the presence of anoxidizing flame to form glass (pure silica) or doped glass particles.Preferably, silicon tetrachloride (SiCl₄) can be used. Alternatively,other silicon containing reactants can be used, such as SiHCl₃, SiH₂Cl₂,SiH₃Cl or SiH₄. In addition chlorine-free silicon containing reactantscan be used, such as the siloxane compounds disclosed in U.S. Pat. No.5,043,002, e.g. octamethylcyclotetrasiloxane, or the organosiliconecompounds disclosed in European Patent Application Publ. No. EP1,016,635, e.g. hexamethyldisilane.

A preferred glass precursor material capable of forming doped glassparticles under the reaction conditions of a flame burner according tothe invention is GeCl4) Germanium tetrachloride. Alternative dopantmaterials are POCl₃ or BBr₃.

Mixtures of the above glass precursor materials (e.g. SiCl₄ and GeCl₄)in variable proportion can be used to suitably modify the refractiveindex of the produced preform.

As the above glass precursor materials are generally liquid at ambienttemperature, they may be fed as liquids to the metal pipes of the burneror they may be preferably vaporized in advance, so that high temperaturevapors of the glass precursor material are flown through the centralpipe of the burner.

As previously mentioned it may be advantageous, in particular forrelatively large dimension burners (e.g. cladding burners), to add apredetermined amount of a high thermal diffusivity gas to the flow ofglass precursor material, in order to increase the heat transfer fromthe flame towards the inner core of said flow.

The thermal diffusivity of a gas is defined as the ratio of the thermalconductivity to the heat capacity. It measures the ability of a materialto conduct thermal energy relative to its ability to store thermalenergy. Typical values of thermal diffusivity of gases can be found on anumber of reference books, such as R. B. Bird, “Transport Phenomena”,Wiley & Sons, New York 1960, or F. P. Incropera, D. P. DeWitt,“Fundamentals of heat and mass Transfer”, Wiley and Sons; 3rd edition,New York, 1996.

For the purposes of the present invention, a high thermal diffusivitygas is a gas having a thermal diffusivity of at least 3.0·10⁻⁵ m²/s orhigher, e.g. up to about 2.0·10⁻⁴ m²/s (values at 400° K.). Examples ofsuitable high thermal diffusivity gases are oxygen, nitrogen, argon,helium or hydrogen, having a thermal diffusivity at 400° K. of 3.6·10⁻⁵m²/s, 3.7·10⁻⁵ m²/s, 3.8·10⁻⁵ m²/s, 3.0·10⁻⁴. m²/s and 2.3·10⁻⁴ m²/s,respectively.

As the thermal diffusivity of a gas depends, further from its specificthermal diffusivity coefficient, also from the mass fraction of theadded gas, it is preferable to use gases with a higher molecular weight,in order to reduce the volume fraction of added gas (or, alternatively,using the same volume fraction of gas, increase its mass fraction).Oxygen is thus preferred for its higher molecular weight and for itsrelatively high coefficient of thermal diffusivity.

Said high thermal diffusivity gas should preferably be added to the flowof glass precursor material in an amount such that the overall thermaldiffusivity of the so obtained mixture is about 50% higher than thethermal diffusivity of the glass precursor material. In particular, whensilicon tetrachloride is used, the thermal diffusivity of the mixtureshould preferably be higher than about 4.0·10⁻⁶ m²/s at 400° K.Preferably, the thermal diffusivity of the mixture is comprised between4.0·10⁻⁶ m²/s and 5.5·10⁻⁶ m²/s at 400° K.

The high thermal diffusivity gas is preferably admixed in a volumefraction of from about 0.05 to about 0.5 parts with respect to the totalvolume of the mixture, preferably of from about 0.1 to about 0.4 parts,depending also from the thermal diffusivity of the glass precursormaterial (e.g. 2.84·10⁻⁶ m²/s at 400° K. for SiCl₄).

A combustible gas and a combustion sustaining gas are then flown throughthe annular ducts of the burner formed by the co-axial metal pipes,optionally together with an inert gas. Examples of suitable combustiblegas are hydrogen or hydrocarbons, such as methane. Oxygen is typicallyused as the combustion sustaining gas.

If desired, an inert gas may be flown through the annular ducts, eitheralone or admixed with the above combustible gas or combustion sustaininggas. For instance, an inert gas may be flown through an annular ductdisposed between a first annular duct dedicated to the inlet of acombustible gas and a second annular duct dedicated to the inlet of acombustion sustaining gas. This allows a physical separation of the twoflows of combustible gas and of combustion sustaining gas, thusdisplacing the flame away from the tips of the metal pipes and avoidingpossible overheating of the same. Similarly, the flame can be displacedaway from the tips of the metal pipes by suitably increasing the inletspeed of the combustible gas and of combustion sustaining gas. Examplesof suitable inert gases are argon, helium, nitrogen.

With specific reference to FIG. 1, the glass precursor material, e.g.silicon tetrachloride, preferably admixed with oxygen, is flown throughthe central opening 101 a, hydrogen through opening 102 a and oxygenthrough opening 103 a of the inner section of the burner. The flows ofhydrogen and oxygen are maintained at a sufficiently high rate in orderto slightly move the flame away from the tips of the metal pipes. Oxygenis preferably kept in a relatively high stoichometric excess withrespect to hydrogen, the O₂/H₂ molar ratio being preferably of fromabout 1.8:1 to about 3:1.

Said excess of oxygen allows to obtain a convergent flame and to createan oxygen boundary layer on the inner surface of the quartz tube 109,for reducing the heat transferred onto the quartz tube. For determiningthe excess of oxygen in the inner flame, also the possible reaction ofsaid oxygen with the hydrogen flowing from the outer section of theburner shall be taken into account. In order to effectively create saidboundary layer, the Applicant has observed that the inlet speed of theoxygen gas into the burner should preferably be of at least 3.0 m/s orhigher, e.g. up to about 10.0 m/s.

In the outer section of the burner, argon is flown through opening 104a, hydrogen through opening 105 a, argon through opening 106 a andoxygen through opening 107 a. In this case, oxygen is flown in astoichometric ratio or preferably in slight excess with respect tohydrogen, the O₂/H₂ molar ratio being from about 1:2 to about 1:1preferably from about 1:1.95 to about 1:1.75.

As previously mentioned, the hydrogen flowing from the outer section mayalso partially react with the excess of oxygen flowing from the innersection of the burner.

According to an alternative configuration shown in FIG. 4, the glassprecursor material, e.g. silicon tetrachloride, preferably admixed withoxygen, is flown through the central opening 401 a, argon throughopening 402 a, hydrogen through opening 403 a, argon through opening 404a and oxygen through opening 405 a of the inner section of the burner.In this case, the interposition of an argon's flow between oxygen's andhydrogen's flows allows the flame to be displaced away from the tips ofthe metal pipes. As previously mentioned, oxygen flowing through opening405 a is preferably kept in a relatively high stoichometric excess withrespect hydrogen. In the outer section of the burner, argon is flownthrough opening 406 a, a mixture of hydrogen and argon is flown throughopening 407 a, and oxygen through opening 408 a. Premixing argon withhydrogen increases the momentum of the mixture containing combustiblegas. This is useful for directing the flow of combustion products towardthe target soot and also for lifting the flame from burner orifices.

The Applicant has further observed that by suitably redistributing theflow of forming glass particles before said flow impacts onto the targetpreform, it is possible to further increase the deposition rate of theburner. To this end, a multi-flame burner as disclosed previously isparticularly suitable. In particular, the outlet of the quartzseparating tube is suitably modified so to increase the deposition rateof the burner. The modification is such as to confer to the outlet ofthe quartz separating tube a cross-section having a major and a minoraxis.

As a matter of fact, the Applicant has observed that for obtaining anoptimal and homogeneous heating of the reacting glass precursormaterial, both the stream of glass precursor material and thesurrounding inner flame shall preferably have a substantially circulargeometry. On the other side, it has been observed that the depositionrate can be increased by increasing the dimensions of the flow of glassparticles in a direction substantially perpendicular with respect to thelongitudinal axis of the target preform.

FIG. 5 show a schematic longitudinal cross-sectional view of a burnerwith a modified glass quartz tube 501 according to the presentinvention, and the relative target preform 505 (not in scale).

FIG. 6 is a section according to plane VI-VI of FIG. 5, showing thelongitudinal cross-section of the terminal portion of the modifiedquartz glass tube 501 and of the relative target preform.

As shown in FIG. 5, the quartz glass tube 501 confining the inner flamehas preferably a substantially circular cross-section in its initialportion 502, i.e. in the proximity of the metal co-axial pipes, andpreferably in its middle portion 503, for causing an optimal andhomogeneous heating of the reacting glass precursor material. Theconfining quartz glass tube 501 is then suitably modified incorrespondence with its terminal portion 504, in order to confer asubstantially elliptical cross section to the flow of glass particlesand to the surrounding flame, with a major axis “A” (see FIG. 6) and aminor axis “B”. Other suitable forms having a major and minor axis (e.g.rectangular) can be applied to the outlet of the glass tube. As observedby the Applicant, the deposition rate of a burner can be increased bydisposing said confining quartz glass tube such that the major axis “A”of the elliptic flow lays on a plane which is substantiallyperpendicular to the longitudinal axis of the preform.

This redistribution of the stream of growing silica particles results inan increase of the deposition rate of the burner.

In order to effectively increase the deposition rate of the burner, theratio between the major axis and the minor axis shall preferably be atleast about 1.2 or higher. On the other side, in order to avoid anexcessive modification of the geometry of the flow of glass particles(which may cause undesired turbulences in the flows of the burner) saidratio is preferably kept lower than about 2.5. Preferably, said ratio isfrom about 1.25 to about 1.8.

In addition, said major axis should be relatively smaller with respectto the initial diameter of the growing preform, in order to avoidexcessive dispersion of silica particles. Preferably, said major axis isin a ratio of at least about 1:2 or higher with respect to the initialdiameter of the growing preform, more preferably of at least about 1:2.5or higher. On the other side, said major axis should be sufficientlylarge with respect to the final diameter of the preform, in order toeffectively increase the deposition rate of the process.

In particular, the ratio between said major axis and the final diameterof the preform is preferably lower than about 1:7, preferably lower thanabout 1:6.

For instance, in a double burner overcladding deposition process asillustrated in FIG. 7, where the upper burner 704 starts the depositionon a growing preform of about 90-100 mm diameter and increases thediameter of said preform up to about 180-200 mm, the cross-sectionaldimensions of the separating quartz tube may be the following:

-   -   Circular section: diameter about 31 mm    -   Elliptical section: major axis about 34 mm; minor axis about        24.5 mm.

The burner of the present invention is particularly suitable for beingused in the overcladding deposition of large diameter preforms, wherethe flow rate of the glass precursor material is typically kept higherthan about 8 slm (standard liter per minute), in particular at about 10slm or higher.

FIG. 7 schematically illustrates a typical overcladding depositionprocess for embodying the method of the present invention. Thedeposition typically starts onto a glass rod 701 of about 20 mmdiameter, comprising the core of the preform and a first portion of thecladding layer, separately manufactured according to conventionaltechniques. The target preform is rotated about is longitudinal axis andslowly upwardly translated. A lower overcladding burner 703 deposits afirst portion of overcladding layer 702 a, e.g. up to a diameter ofabout 90-100 mm onto the preform. An upper burner 704 then completes thedeposition by depositing a second overcladding layer 702 b, e.g.increasing the diameter of the deposited soot at about 180-200 mm.Typically, the upper burner 704 has increased dimensions with respect tothe lower one, in order to allow the deposition of higher amount ofsilica particles in the time unit. These dimensions correspondsubstantially to the dimensions mentioned in connection with the burnerillustrated in FIGS. 1-4.

The so obtained preform is then heated into a furnace and collapsed toobtained a final preform of about 60-80 mm diameter, which is then drawninto an optical fiber according to conventional techniques.

While a burner according to the present invention can advantageously beused in the above process for depositing the overcladding layer of thepreform, in particular the outer overcladding portion (i.e. as burner704), it will be appreciated that such a burner, when suitablydimensioned, can also be used for the deposition of the core and of theinner portion of the cladding.

EXAMPLE 1 Effects of Flame Confining Quartz Tube

For this experiment, a burner comprising eight co-axial metal pipes asshown in FIGS. 1 and 2 has been used. The material used for the metalpipes was AISI316L stainless steel. Pipes 101-108 and ducts 10 a-107 aof FIG. 1 will be referred to in the present example as pipes 1-8 andducts 1 a-7 a, respectively.

A quartz glass tube has been inserted between the third and the fourthmetal pipe for providing the flame confinement. The following table 1indicates the relative internal (ID) and outer (OD) diameter of theannular ducts determined by the metal pipes; for the innermost duct 1 a,having a circular cross section, only the OD has been reported. Theinner section of the burner is formed by pipes 1 to 3 (and correspondingducts 1 a to 3 a), while the outer section of the burner is formed bypipes 4 to 8 (and corresponding ducts 4 a to 7 a)

TABLE 1 dimensions of ducts Duct no. 1a 2a 3a 4a 5a 6a 7a ID (mm) — 1121.34 37.6 44.2 55.8 61.1 OD (mm) 7 17.6 24.4 40.2 50.5 58.3 67.55

The internal confining quartz glass tube, having a thickness of about1.5 mm, an inner diameter of 28.4 mm and an outer diameter of 31.4, hasbeen inserted into the annular clearance between pipes 3 and 4 (ID 27.4mm, OD 33.6 mm). The lower portion of the glass tube has been wrappedwith a Teflon® tape up to the outer diameter of the clearance, in orderto maintain it in a fixed position.

An outer quartz glass tube having a thickness of about 2 mm has beenfurther disposed around the outer metal pipe 8.

As shown in FIG. 2, all the metal pipes were substantially of the samelength. The outer quartz tube protruded for about 165 mm from the tipsof the metal pipes, while the length of the internal quartz tube hasbeen varied, as shown in table 3.

The reactants employed and their relative flow rate and inlet speed arereported in the following table 2, where the innermost opening of theburner is identified with no. 1 a. Silica tetrachloride has beensupplied by vaporizing the liquid material and feeding it at atemperature of about 80° C. through the central pipe, together withoxygen.

TABLE 2 Reactants and flow rate Duct no. 1a 2a 3a 4a 5a 6a 7a ReactantSiCl₄ + O₂ H₂ O₂ Ar H₂ Ar O₂ Flow Rate 12 + 7 27 65 14 160 10 115 (slm)Inlet velocity 8.2 3.4 9.9 1.5 5.7 0.7 2.9 (m/s)

Under these conditions, a theoretical length of the reaction zone ofabout 100-120 mm has been calculated, according to the relationshippreviously illustrated.

The target preform was a rotating quartz tube of about 90 mm diameterand the burner (i.e. the upper end of the outer glass tube of theburner) has been kept at a distance of about 50 mm from the preform,with an inclination of about 12° with respect to the longitudinal axisof the preform.

The preform was translated upwardly at a speed of 168 mm/h and rotatedat about 60 r.p.m.

The deposition was stopped when the preform reached a diameter of about140-150 mm.

By increasing the protruding length of the inner confining quartz tubefrom the tips of the metal pipes, an increase in the deposition rate hasbeen observed, as reported in the following table 3. The deposition ratereported in table 3 is normalized with respect to the deposition rate ofa burner without separating quartz tube (length 0 mm).

TABLE 3 Deposition rate Tube protruding Normalized length (mm)deposition rate 0 1 10 1.24 27 1.47 47 1.56 67 1.70 87 1.74 107 1.77 1271.79

From the results reported in the above table 3, it can be observed thata tube with a length of 87-107 mm provides a substantial increase in thedeposition rate. When using the longer tube (127 mm), although a slightincrease in the deposition rate has still been observed, the resultingpreform showed cracks in the deposited soot, which indicates a notappropriate density value. This is possibly caused by the fact that theoutlet of the separating tube was positioned too close to the targetpreform, thus concentrating the inner flame onto a too small surface ofthe preform.

Some other experiments have thus been carried out by positioning theburner further 40 mm away from the target soot, and the recipe has beenmaintained as per table 2. Corresponding to three different separatingtube lengths of 107, 133 and 153 mm, the obtained normalised depositionrates were 1.81, 1.89 and 1.83, respectively.

At the end of the deposition process, no damages or deformation havebeen observed onto the metal pipes forming the outer flame of theburner.

EXAMPLE 2 Modification of the Cross-Section of the Separating QuartzTube

For this experiment a burner having a configuration according to FIG. 4has been used. Pipes 401-409 and ducts 401 a-408 a of FIG. 1 will bereferred to in the present example as pipes 1-9 and ducts 1 a-8 a,respectively.

Dimensions of ducts created by pipes 1-9 and the relative flow ofmaterials is reported in tables 4 and 5, respectively.

TABLE 4 Dimensions of the burner Duct No 1a 2a 3a 4a 5a 6a 7a 8a ID (mm)0 8 10 18 20 32.4 39 50 OD (mm) 7 9 17 19 27 37 48 55

TABLE 5 Flow of reactants Duct No 1a 2a 3a 4a 5a 6a 7a 8a ReactantSiCl₄ + O₂ Ar H₂ Ar O₂ Ar H₂ + Ar O₂ Flow Rate (slm) 11 + 3 0.8 22 1.650 10 118 + 21 60

The internal separating quartz glass tube, having a thickness of about1.5 mm, has been inserted into the annular clearance between pipes 5 and6.

An outer quartz glass tube having a thickness of about 2 mm has beenfurther disposed around tube the outer metal pipe 9.

In this experiment, the tips of pipes 6 to 9, forming the outer sectionof the burner, were spaced forwardly of about 53 mm from the tips of thepipes 1 to 5, forming the inner section of the burner.

The inner quartz separating tube was prolonged for about 70 mm from thetips of the pipes of the outer section, having thus a total length ofabout 123 mm from the tips of the pipes of the inner section.

The target preform was a rotating quartz tube of about 90 mm diameterand the burner has been kept at a distance of about 90 mm from thepreform, with an inclination of about 12° with respect to thelongitudinal axis of the preform.

The preform was translated upwardly at a speed of 168 mm/hr and rotatedat about 60 r.p.m.

The deposition was stopped when the preform reached a diameter of about140-150 mm.

The above burner has then been modified by conferring an ellipticalcross-section to the terminal portion of the separating quartz tube, asshown in FIGS. 5 and 6 . The section has been changed from circular intoelliptical at about 10 mm from the end of the tube, by maintainingsubstantially unaltered the fluid passage area of the tube. The majoraxis of the internal elliptical cross-section at the outlet of the tubewas of about 34 mm, while the minor axis was of about 24.6 mm.

The burner has then been positioned similarly to the previous test, withthe major axis of the elliptical cross-section of the quartz tube layingon a plane substantially perpendicular to the longitudinal axis of thetarget preform. The process conditions were as previously described.

A normalized deposition rate (with respect to the value obtained withthe circular cross-section burner) of about 1.08 was obtained.

As a comparative experiment, the above burner has been rotated of 90°with respect to its own longitudinal axis, so to exchange the relativepositions of the major and of the minor axis. In this case, a depositionrate of about 0.76.

As shown by the above results, it is thus possible to increase thedeposition rate of a burner according to the invention by suitablymodifying the cross-section of the outlet of the separating quartz tube.

1. A method for manufacturing an optical preform by directing a flow offine glass particles from a deposition burner onto a rotating elongatedtarget preform, said method comprising the steps of: (a) feeding saidburner with a flow of a glass precursor material via a first pluralityof ducts and directing said flow of glass precursor material toward saidtarget preform; (b) feeding said burner with a first flow of combustiblegas and a first flow of combustion sustaining gas via said firstplurality of ducts in order to generate an inner flame surrounding saidflow of glass precursor material; (c) feeding said burner with a secondflow of combustible gas and a second flow of combustion sustaining gasvia a second plurality of ducts in order to generate an outer flamesurrounding said inner flame; (d) reacting said glass precursor materialin the presence of said flames, thus forming a flow of fine glassparticles directed toward said target preform; and (e) confining andseparating said inner flame from said outer flame with an elongatedhollow separating element made of a heat resistant material thatsurrounds said inner flame and that does not form a part of a duct forthe passage of any gas or glass precursor material.
 2. A methodaccording to claim 1, wherein said elongated hollow separating elementhas a length greater than the length of said first plurality of ducts.3. A method according to claim 1, wherein said elongated hollowseparating element has a length greater than the length of said secondplurality of ducts.
 4. A method according to claim 1, further comprisingthe step of confining said outer flame with a second elongated hollowseparating element made of a heat resistant material that surrounds saidouter flame and does not form a part of said second plurality of ducts.5. A method according to claim 1, further comprising the step ofassembling said first plurality of ducts and said second plurality ofducts so as to leave a clearance to receive said elongated hollowseparating element.